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# Electrical Riddle No.44 – Two ac sources in parallel

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Kotteeswaran

If I need to connect two ac sources in parallel, what are the things I have to consider?
If I need to connect two ac sources in parallel, what are the things I have to consider?

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Hamid

For interconnection two power sub system, e.g. a utility system and a distributed generation unit some important subject shall be considered as following: A- Insulation Coordination Any equipment, including distributed generators, connected to the utility system (even for brief periods) must be able to withstand the normal and abnormal voltages that can be experienced on the utility system. With a properly designed distribution system, these normal and abnormal voltages should not cause damage to or reduce the reliability of any connected equipment. This statement applies to all connected equipment regardless of ownership including the utility, distributed generator, or other customer Utility equipment is designed with a certain basic impulse insulation level (BIL). BIL, in general terms, is a measure of the ability of a piece of equipment to withstand normal and abnormal voltages. Lightning and switching of equipment are two common sources of high abnormal voltage transients. For example The US 13.8 kV system is designed for 95 kV BIL, the standard level for this voltage class. The US distribution system is in a geographical area subject to significant lightning. US line equipment is protected by utility-grade distribution surge arresters rated at 12 kV, which is the normal recommended rating for 13.8 kV multi-grounded neutral distribution systems. 12 Kv arresters have a rated discharge voltage of 39 kV for a standard 10 kA 8×20 sec lightning test waveform for which arresters are rated. Thus, a 12 kV arrester with this discharge voltage would achieve a 143% protection margin, PM, for 95 kV BIL equipment: This has proven to be a reliable margin and is typical for utility distribution systems of this voltage class. This margin is necessary for several reasons, including: – Equipment BIL declines as it ages. Having a large protective margin allows the equipment to remain in service longer. – Many lightning stroke currents are larger than 10 kA. For example, a 40 kA 8×20 sec lightning test wave will yield a discharge voltage of 48 kV in a standard 12 kV arrester. – Many lightning strokes have a much steeper rate-of-rise than the standard 8×20 sec test waveform and produce significantly higher discharge voltage across the arrester and the leads connecting the arresters to the system. – Lightning surge voltages tend to double when they reach the end of a cable or an overhead line. These factors shrink the protective margin considerably. In contrast to standard utility line equipment, the protective margin for rotating machine generators proposed for DG applications to be directly connected to the US system is often very low in comparison. Depending on the current magnitude and the rate-of-rise of an incoming surge, the margin provided by US arresters will be insufficient to protect these machines for many of the voltage levels expected to be induced from common lightning strikes to the distribution system. One might think that applying an arrester with a lower discharge voltage rating might protect such machines. However, this approach would result in these arresters attempting to protect the entire distribution system for some abnormal, but expected, low frequency surge phenomena that might occur. Examples of the latter condition include unfaulted phase voltage rise during single line-to-ground faults and resonant conditions that can arise on phase conductors. The neutral grounding reactance in the substations, while limiting the ground fault current, increases the unfaulted phase voltage rise. The voltage rise on unfaulted phases can subject arresters to excessive transient overvoltage (TOV) duty, causing the arrester to fail and isolate itself from the system. Unfortunately, this failure and resulting isolation would leave the machine vulnerable to the next lightning or switching transient overvoltage. Using a standard utility service transformer for interconnecting the generator will generally resolve the insulation coordination issue. Such transformers are designed for this kind of service and provide the necessary separation of insulation levels from the high voltage winding to the low voltage winding. The reader is reminded that it is not possible to guarantee absolute protection from voltages induced by lightning surges. Even with a properly coordinated transformer, lightning may enter the electrical system from another path and induce voltages that cause damage to the machine insulation. B- Underground Cable Express Feed US may choose to provide a waiver to the interconnection transformer requirement if 1. The proposed DG site is served by underground cable directly from substation bus, 2. There are no other customers on the feeder (express feed), 3. It can be done safely without other technical issues identified during the feasibility study such as harmonics, overvoltages due to switching transients, and overvoltages during islanding. Such an arrangement minimizes the risk of lightning strikes to the feeder because the substation end is well shielded as presumably would be the generator end of the distribution feeder. A mixed overhead/underground feeder, even if an express feed, can create significant risk of failureif the overhead portion is struck by lightning. The resulting surge will travel long distances down the cable where the voltage will tend to double when the surge reaches the transition in system construction at the generator. An underground express feed does not guarantee immunity from voltages induced by lightning because lightning may strike a nearby structure and induce voltages that enter the generator through other conductors in the electrical system. Lightning has also been shown to occasionally strike buried cables through the soil. Therefore, careful attention to grounding, bonding, and ground referencing must still be given even if the entire system is underground. The express feed arrangement also minimizes the exposure to the impacts of disturbances related to serving other customers. A common complicating factor is the need for an alternate source in case of underground cable failure. In many cases on the US system, this alternate source will be provided by an open-wire overhead system. If the DS is to be operated while connected to such an alternate feed, a waiver cannot be granted. If it is a critical that the DG operate when the main feed has failed, a second dedicated underground cable feed could be considered in lieu of an interconnection transformer. C- Neutral Stability during Islanding The UI distribution system is designed assuming that the source of power provides an

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